The walls of oil and gas formations are exposed during the process of drilling a borehole. The successful completion of a well bore requires the deposit of a low-permeable filter cake on the walls of the well bore to seal the permeable formation exposed by the drilling bit. A filter cake can limit drilling fluid losses from the well bore and protect the natural formation from possible damage by the fluids permeating into the well bore. Solids in the drilling fluid may also damage the formation, particularly drilling fines. The suspension of fine particles that enters the formation while the cake is being established is known as “mud spurt” and the liquid that enters subsequently is known as “filtrate”. Both filtration rate and mud spurt must be minimized when penetrating potentially productive formations because productivity may be reduced by any one of the following: the swelling of clays in the formation when they come in contact with the filtrate; particles transported into the pores of the formation that plug flow channels and greatly reduce the permeability of the rock; and the pressure of some reservoirs that is not great enough to drive all of the aqueous filtrate out of the pores of the rock when the well is brought into production. For a filter cake to form, the drilling fluid must contain some particles of a size only slightly smaller than the pore openings of the formation. These particles are known as bridging particles and are trapped in surface pores, thereby forming a bridge over the formation pores. Filter cake building fluids can also contain polymers for suspension of solids and for reducing liquid loss through the filter cake by encapsulating the bridging particles. These can be either natural or synthetic polymers. The polymers can include one polymer such as xanthan selected for its rheological properties and a second polymer, a starch for example, selected for reduction of fluid loss.
At completion of the drilling or other well servicing, the filter cake must be removed to allow production of the formation fluids or bonding of cement to the formation at the completion stage. Removal of the deposited filter cake should be as complete as possible to recover permeability within the formation.
Previous chemical treatments for filter cake removal have employed an acid to dissolve carbonates and/or hydrolyze polysaccharide polymers. Dobson, Jr. et al., U.S. Pat. No. 5,607,905, reveal a process for enhancing the removal of filter cake by the use of inorganic peroxides as oxidizing agents. The process disclosed in the '905 patent incorporates alkaline earth metal peroxides, zinc peroxides or a mixtures thereof within the filter cake as an integral component thereof and then contacts the filter cake with an acidic solution. Hollenbeck et al., U.S. Pat. No. 4,809,783, disclose a method of dissolving a polysaccharide-containing filter cake present in a subterranean formation. The method comprises injecting an effective amount of a treatment fluid having a water soluble source of fluoride ions present in an amount sufficient to provide a molar concentration of from about 0.01 to about 0.5 and a source of hydrogen ions present in an amount sufficient to produce a pH in the treatment fluid in the range of from about 2 to about 4 into a subterranean formation wherein a filter cake is present. The treatment fluid is maintained within the subterranean formation and in contact with the filter cake for a sufficient time to dissolve at least a portion of said filter cake. In Example 1, Hollenbeck describes the addition of an aqueous solution of sodium persulfate to the fluoride solution.
Several of the disclosed references teach the use of perceived high concentrations of persulfate in a fracturing fluid. In U.S. Pat. No. 5,164,099, Gupta et al., claim a method for breaking an aqueous fracturing fluid comprised of introducing an encapsulated percarbonate, perchlorate, or persulfate breaker into a subterranean formation being treated with the fracturing fluid. The encapsulated breaker is comprised of a polyamide membrane enclosing the breaker, the membrane permeable to a fluid in the subterranean formation such that the breaker diffuses through the membrane to break the fracturing fluid with the membrane staying intact. Gupta further claims the persulfate composition, used as an encapsulated breaker within a frac fluid, to be comprised of ammonium persulfate. In U.S. Pat. No. 4,250,044, Hinkle describes a persulfate frac fluid breaker system for reducing the viscosity of fracturing fluids at temperatures ranging from 50° F. to 125° F. The '044 persulfate breaker system must have an activator, a tertiary amine, with the persulfate for the breaking action to be effective. Breaker fluids for breaking fracturing fluids can comprise concentrations of persulfate ranging from ¼ to about 20 lbs of persulfate per 1000 gals. of fluid, Hinkle, col. 5, lines 66 to 70.
Norman et al. in two patents, U.S. Pat. No. 5,373,901 and U.S. Pat. No. 6,357,527, describe a method of breaking an aqueous fracturing fluid comprising introducing the aqueous fracturing fluid into contact with an encapsulated viscosity reducing agent. The encapsulated viscosity reducing agent comprises an aqueous fluid soluble breaker for the fracturing fluid encapsulated within a membrane comprising a partially hydrolyzed acrylic crosslinked with either an aziridine prepolymer or a carbodiamide. The membrane has an embrittlement effective amount of a micron sized particulate present therein and has been cured at a temperature of at least about 115° F. such that an aqueous fluid in the fracturing fluid can contact the breaker after fracture failure of the membrane to dissolve at least a portion of the breaker and break the fracturing fluid in contact therewith.
Todd, in a patent application publication, US 2002/0036088, claims a well drilling or servicing fluid for use in producing formations. The fluid deposits a filter cake and contains water, a water soluble salt and a particulate solid bridging agent. The improvement is the particulate solid bridging agent comprising a chemically bonded ceramic oxychloride cement, magnesium oxysulfide cement, magnesium potassium phosphate hexahydrate, magnesium hydrogen phosphate trihydrate and magnesium ammonium phosphate hexahydrate. The resulting filter cake is dissolvable by an aqueous clean-up solution containing a mild organic acid, a hydrolyzable ester, an ammonium salt, a chelating agent or a mixture of an ammonium salt and a chelating agent. The '088 application suggests the use of ammonium persulfate incorporated into the bridging agent, see pgr. 0025.
Mondshine in U.S. Pat. No. 5,253,711 teaches a process for decomposing polysaccharides in alkaline aqueous systems. The process comprises using alkline earth metal or transition metal peroxides as a delayed breaker. The alkaline aqueous fluids contain a water soluble hydrophilic polysaccharide polymer hydrated within. The peroxide is activated by increasing the temperature of the fluid.
In the case of horizontal open hole drilling of unconsolidated formations, it is desirable to gravel pack the well bore after drilling the zone but before the filter cake is completely removed. The act of gravel packing the well bore annulus further limits fluid contact with the filter cake, as it both reduces the physical volume of fluid that can be present in the zone and restricts direct flow to the filter cake. As a consequence, the effectiveness of the breaking of the filter cake is dramatically reduced.
Problems also exist with many of the prior methods of removing filter cake downhole including the problem of controlling the breaking of the filter cake so that production fluids do not enter the well bore before the entire, or at least most, of filter cake is broken down. Breaking down the first portion of the filter cake with a clean-up (or breaker) fluid while the breaker fluid has not reached the remaining areas can cause premature flowing of production fluids or leaking of clean up fluids into the formation. A need exists for clean up fluids that have a delayed effect on filter cake integrity, allowing the breaker solution to be circulated across the interval before leakoff to the formation becomes a problem.